Fix 26 bugs, edge cases, and consistency issues from fifth audit pass

Critical: undo toast now trashes only batch output files (not entire dir),
JPEG scanline write errors propagated, selective metadata write result returned.

High: zero-dimension guards in ResizeConfig/fit_within, negative aspect ratio
rejection, FM integration toggle infinite recursion guard, saturating counter
arithmetic in executor.

Medium: PNG compression level passed to oxipng, pct mode updates job_config,
external file loading updates step indicator, CLI undo removes history entries,
watch config write failures reported, fast-copy path reads image dimensions for
rename templates, discovery excludes unprocessable formats (heic/svg/ico/jxl),
CLI warns on invalid algorithm/overwrite values, resolve_collision trailing dot
fix, generation guards on all preview threads to cancel stale results, default
DPI aligned to 0, watermark text width uses char count not byte length.

Low: binary path escaped in Nautilus extension, file dialog filter aligned with
discovery, reset_wizard clears preset_mode and output_dir.

pixstrip-core/tests/watcher_tests.rs

@@ -54,7 +54,7 @@ fn watcher_detects_new_image() {
     watcher.start(&folder, tx).unwrap();
 
     // Wait for watcher to be ready
-    std::thread::sleep(std::time::Duration::from_millis(200));
+    std::thread::sleep(std::time::Duration::from_millis(500));
 
     // Create an image file
     let img_path = dir.path().join("new_photo.jpg");
@@ -87,7 +87,7 @@ fn watcher_ignores_non_image_files() {
     let (tx, rx) = mpsc::channel();
 
     watcher.start(&folder, tx).unwrap();
-    std::thread::sleep(std::time::Duration::from_millis(200));
+    std::thread::sleep(std::time::Duration::from_millis(500));
 
     // Create a non-image file
     std::fs::write(dir.path().join("readme.txt"), b"text file").unwrap();